Rhodium triiodide
[CAS# 15492-38-3]

Identification

• Classification: Inorganic chemical industry >> Inorganic salt >> Metal halides and halides >> Metal iodide and salt
• Name: Rhodium triiodide
• Synonyms: Rhodium (III) iodide
• Molecular Formula: RhI₃
• Molecular Weight: 483.62
• CAS Registry Number: 15492-38-3
• EC Number: 239-521-5
• SMILES: [Rh](I)(I)I

Safety Data

• Hazard Symbols: GHS07 Warning
• Risk Statements: H317-H319
• Safety Statements: P280-P305+P351+P338

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Chronic hazardous to the aquatic environment	Aquatic Chronic	4	H413

Discovery and Applications

Rhodium triiodide is an inorganic compound with the chemical formula RhI₃, comprising rhodium in the +3 oxidation state coordinated by three iodide ions. It is a transition metal halide characterized by the combination of a heavy transition metal with large, polarizable halide ligands. Rhodium triiodide typically appears as a dark-colored crystalline solid, often exhibiting colors ranging from deep brown to black, depending on its exact form and preparation method.

The compound can be prepared by direct combination of elemental rhodium metal and iodine under controlled heating conditions or by the reaction of rhodium(III) chloride with excess potassium iodide in aqueous or alcoholic media, leading to halide exchange and precipitation of rhodium triiodide. Another synthetic approach involves treatment of rhodium metal salts with hydroiodic acid or iodine-containing reagents under reductive or acidic conditions. The product generally requires purification by recrystallization or sublimation due to the sensitivity of iodide complexes to moisture and air.

Structurally, rhodium triiodide adopts an octahedral coordination geometry around the rhodium center in its crystalline lattice, with Rh³⁺ ions coordinated by iodide ligands. In the solid state, these complexes often form polymeric networks stabilized by bridging iodide ions, which contribute to the compound’s stability and unique physical properties. The large ionic radius and high polarizability of iodide ions influence the bonding character, giving the compound a combination of ionic and covalent characteristics.

Rhodium triiodide is primarily used as a precursor in coordination and organometallic chemistry. It serves as a source of rhodium(III) for synthesizing various rhodium complexes with applications in catalysis and material science. Due to the strong ligand field exerted by iodide ions, RhI₃ is a useful starting material for preparing low-spin rhodium complexes with tailored electronic and steric environments. These complexes often exhibit interesting photophysical, redox, and catalytic properties.

In homogeneous catalysis, rhodium complexes derived from rhodium triiodide have been explored in a range of reactions including hydrogenation, hydroformylation, and carbon–carbon bond-forming transformations. While rhodium chloride complexes are more common in industrial catalysis, iodide ligands can impart distinct reactivity and selectivity, especially in cases where softer donor ligands or unusual electronic effects are desirable. The iodide substituents can influence the catalytic cycle by modulating electron density at the metal center and altering substrate binding affinities.

Beyond catalysis, rhodium triiodide has been investigated for its role in the synthesis of novel materials. It is used as a precursor for preparing rhodium-containing nanoparticles and thin films through chemical reduction or decomposition methods. Such materials are valuable for their catalytic, electronic, and optical properties, including applications in sensors, electronics, and energy conversion devices.

The compound is sensitive to moisture and air, as iodide ligands can be oxidized or replaced under certain conditions. Thus, rhodium triiodide is generally handled under inert atmosphere and stored in airtight containers to maintain its integrity. It is moderately toxic and should be handled with appropriate protective equipment to avoid exposure via inhalation, ingestion, or skin contact.

In summary, rhodium triiodide is a dark crystalline rhodium(III) halide with iodide ligands coordinated octahedrally to the metal center. It serves as a useful precursor in coordination chemistry and catalysis, contributing to the development of rhodium-based catalytic systems and functional materials. Its distinct ligand environment imparts unique chemical and physical properties, making it a compound of interest in both fundamental and applied inorganic chemistry.

References

2022. Study on Rh(I)-o-aminophenol Catalyst Catalyzed Carbonylation of Methanol to Acetic Acid. Arabian Journal for Science and Engineering, 47(12).
DOI: 10.1007/s13369-022-06936-w

2004. Carbonylation of Methanol and Methyl Acetate. Homogeneous Catalysis.
DOI: 10.1007/1-4020-2000-7_6

1983. Catalytic carbonylation of nitrobenzyl- and nitroarylpyridines. Chemistry of Heterocyclic Compounds, 19(1).
DOI: 10.1007/bf00512819